1. Field of the Invention
The present invention relates to a brake by wire (BBW) system which electrically operates a fluid pressure generator using an electrical signal which is outputted in response to a brake operation of a driver. The brake system operates a wheel cylinder using brake fluid pressure generated by the fluid pressure generator in order to brake a wheel.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2000-127805 discloses a so-called BBW type brake system which normally operates a hydraulic brake using brake fluid pressure generated by a power fluid pressure source when communication between a master cylinder that generates brake fluid pressure by a driver depressing a brake pedal and a hydraulic brake (wheel cylinder) that brakes a wheel is shut off by means of a master cylinder cut valve (shutoff valve). During such normal operation the power fluid pressure source (fluid pressure generator) that generates brake fluid pressure is operable. The brake system opens the master cylinder cut valve to operate the hydraulic brake with the brake fluid pressure generated by the master cylinder during abnormal operation, having previously absorbed the brake fluid pressure generated by the master cylinder during the above described normal operation using a stroke simulator. The stroke simulator enables a stroke of the brake pedal during abnormal operation when the power fluid pressure source becomes inoperable.
If the conventional BBW type brake system has a power fluid pressure source comprising a fluid pressure generator which generates brake fluid pressure by driving a piston using an electric motor in such a BBW type brake system, the fluid pressure generator and a reservoir of the fluid pressure generator are allowed to communicate with each other by a special fluid passage. The reservoir of the master cylinder is also used as the reservoir of the fluid pressure generator. In this structure, when the fluid pressure generator fails, a master cylinder cut valve is opened, and the wheel cylinder is operated with the brake fluid pressure generated by the master cylinder. Therefore, an atmosphere valve provided in the special fluid passage is required to be closed in order to prevent the brake fluid pressure generated by the master cylinder from escaping to the reservoir through the special fluid passage.
However, if the atmosphere valve is employed in the brake system, the number of components and cost correspondingly increase, and therefore it is desirable to eliminate the atmosphere valve while maintaining the function of the BBW type brake system.